Re: Question about gpio sysfs interface

[Alexandre Courbot <gnurou@gmail.com>]

On Mon, Apr 7, 2014 at 3:56 AM, Michael Heimpold <mhei@heimpold.de> wrote:
> Hi,
>
> I have a question regarding user-space gpio support with sysfs.
>
> Documentation/gpio/sysfs.txt states
>> GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
>> and have the following read/write attributes:
>>
>>    /sys/class/gpio/gpioN/
>>
>>       "direction" ... reads as either "in" or "out". This value may
>>               normally be written. ...
>>
>>               Note that this attribute *will not exist* if the kernel
>>               doesn't support changing the direction of a GPIO, or
>>               it was exported by kernel code that didn't explicitly
>>               allow userspace to reconfigure this GPIO's direction.
>
> Imagine for example a generic user-space tool which enumerates
> all available GPIOs to the user and sorts by input and output GPIOs.
> How would such a tool classify an exported GPIO when kernel
> doesn't export the 'direction' file via sysfs? Or in other words: how
> can userspace detect the state (in vs. out) of a GPIO when 'direction'
> is not exported?

Wouldn't the "value" field be read-only for a "in" GPIO and read-write
for an "out" one?

Other than that, I also don't see any drawback to having "direction"
always exported and made read-only if direction cannot be changed.

Alex.